The reproductive epidemiology program emphasizes the development of new methods for measuring and analyzing human reproductive outcomes. Such outcomes include fertility, sub-clinical early fetal loss, spontaneous abortion, fetal growth, and birth weight. Each of these outcomes is affected by environmental factors, and represents a likely endpoint for studying the effects of toxins on human reproduction. Applied problems of measuring early fetal loss are being pursued in a joint project with NICHD. A recently-developed assay for human chorionic gonadotropin will be used to monitor pregnancies in conceptable women. In another current study, the validity of measuring spontaneous abortion risk by interviewing women is being evaluated. More theoretical problems in the analysis of spontaneous abortion risk and of birth weight are being approached through the development of mathematical models. Finally, age-at-menarche, seasonality, and other factors are being studied for their effect on reproductive outcomes.